Some embodiments described herein relate generally to methods and apparatus for improving the performance of a coherent optical transponder in an optical communication system. In particular, but not by way of limitation, some embodiments described herein relate to methods and apparatus for improving the skew tolerance of a coherent optical transponder in an optical communication system.
With a growing demand of optical communication systems with high data rates capability, optical quadrature amplitude modulation (QAM) signals are generated to provide high data-carrying capacity and high spectral efficiency. Quadrature amplitude modulation (QAM) is a modulation technique where two or more binary or multi-level electrical data signals are modulated, via an in-phase, or “I” channel, and a quadrature (90 degree) phase, or “Q” channel, onto a single optical carrier wave such that both the amplitude and the phase of the optical carrier wave are modulated with data to enhance the efficiency of the spectral occupancy. Other modulation techniques include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), differential quadrature phase-shift keying (DQPSK), and on-off keying (OOK). Polarization multiplexing (PM) is a multiplexing technique where two independent electrical data signals are first modulated onto an optical carrier wave having orthogonal polarizations (e.g., a first electrical data signal is modulated onto an X channel polarization and a second electrical data signal is modulated onto a Y channel polarization), then the signal on two polarizations are further multiplexed together through a polarization beam combiner so that the overall data throughput is doubled without doubling the spectral bandwidth.
A typical dual-polarization QAM (DP-QAM) transponder includes four tributary channels, XI, XQ, YI, and YQ, which are used for in-phase and quadrature modulation for both an X channel polarization and a Y channel polarization. During propagation of an electrical signal (e.g., a DP-QAM signal, a DP-QPSK signal, and/or the like) and due to material defects of each optical modulator (and other factors such as temperature change, material deterioration over time), a skew may occur between the I channels of the electrical signal and the Q channels of the electrical signal (e.g., between the XI and XQ channels and/or between the YI and YQ channels; collectively referred to as an IQ skew). When uncompensated (e.g., when the optical signal remains skewed), the IQ skew may degrade network performance for a high data rate optical communication system (e.g., a 400 gigabit per second (Gb/s) system).
Known solutions include compensating the IQ skew during an initial calibration of an optical transponder by using a test data pattern. These solutions, however, rely on the specific test data pattern and cannot be implemented with live traffic. As the live traffic often differ from the test data pattern and the IQ skew varies over a change in temperature and time, a need exists for methods and apparatus to compensate the IQ skew accurately with live traffic and improve the tolerance of the IQ skew in the optical communication system.